Cartridge based tape and disk data storage devices have been in use in the computer industry for several decades. Primarily, the cartridge based tape storage devices have been used as sequential access devices, whereby new files are added to the tape by appending them to the last file stored on the tape. During that time, a number of tape cartridge styles emerged. One popular cartridge style is based on a design that is disclosed in U.S. Pat. No. 3,692,255 (Von Behren). That design contains two rotatable reels that are fixed within a rectangular housing. A length of tape is wound around the reels along a predetermined tape path, which extends along a front peripheral edge of the cartridge and across a tape access opening. A drive belt extends around drive belt rollers and contacts a portion of the tape on each reel to move the tape back and forth between the reels and across the tape access opening. A drive puck, positioned near the inside front of the cartridge, contacts a drive roller, which provides a mechanism to move the drive belt.
Tape cartridges and tape drives have become an increasingly important feature in computer systems. The popularity of tape cartridges is driven in part by the large storage capacities and low cost of storage they provide. In general, the storage capacity of a particular tape cartridge is dictated by a variety of factors including the length of tape, the width of the tape, the materials used to produce the tape and the recording density of the tape. Even with the cost and capacity advantages offered by tape storage solutions, the tape drive has primarily found use in computer systems as a back-up device, in which duplicates of files that were originally stored to a random access storage device, such as a hard disk drive, are stored for sequential access on a tape cartridge.
The popularity of these tape drives and cartridges have spawned several tape drive and cartridge standards. One popular tape drive and cartridge standard is defined in the specification entitled "QIC-3020-MC" Revision H, Mar. 20, 1996. That standard defines a variety of important features related to tape cartridges such as tape width, recording format, track format, segment format, and so on.
In a typical hard disk drive back-up procedure, files contained on a hard disk drive are stored on tape in a sequential fashion. That is, each new file is appended after the last file stored on the tape. Thereafter, in the event of a loss of files on the hard drive, the files can be retrieved from the tape and restored to the hard disk drive. Additionally, files that are accessed infrequently can be stored on tape and permanently removed from the hard drive. As a result, valuable hard disk file space is available for more immediate file needs. Because the back-up process can be performed as a background task, the speed of the tape access is generally not the primary concern of the user. Rather, capacity and storage cost are generally more important factors.
While tape storage systems have found substantial use as sequential access storage solutions, the use of tape systems as random access storage solutions has been hindered by slow file access speeds offered by tape storage systems. Tape drive file access speeds are affected by the amount of tape that can be moved across the tape access opening, referred to as tape displacement, and the amount of tape that must be displaced to reach a particular file. Currently available tape cartridges can contain in excess of 1000 feet of tape, and exemplary drives that use cartridges have displacement speeds on the order of about 7 feet per second. Thus, if the tape is positioned near the beginning of the tape and a requested file is near the end of the tape, the access time would take upwards of 140 seconds. For hard disk drive users, accustomed to access times on the order of milliseconds, 140 second access times are unbearable.
Throughput during back-up or restoration is dependent on the switching data rate of the tape drive and controller. During the back up or restoration of a small data file, the software in a computer typically is not able to keep up with the speed of the drive. Thus, the tape drive must stop to allow the computer to catch up. After the computer has caught up, the drive re-starts and needs to relocate. The starting and stopping of a tape drive between blocks of data is wasted time. The relocation of the drive upon re-start takes several seconds, which greatly affects the back-up or restoration time. During a back-up or restoration of many small data files, the relocation occurs many times, further delaying the back-up or restoration operation.
Although the art of tape drives is well developed, there remain some problems inherent in this technology, particularly with respect to the back-up or restoration time. Therefore, a need exists for a back-up and restoration method and apparatus that overcomes the drawbacks of the prior art.